Ultrasonic amplifiers. oscillators, circulators, isolators
and switches



Re. 26,091 ISOLATORS Sept. 20. 1966 D. L. WHITE ULTRASONIC AMPLIFIERS.OSCILLATORS. CIRCULA'I'ORS AND SWITCHES 8 Sheets-Sheet 5 Original FiledApril 26. 1961 FIG. 6

FIG. 7

INVENTOR. D- L. WHITE BY 5? ATTORNEY United States Patent 26,091ULTRASONIC AMPLIFIERS, OSCILLATORS, CIR- CULATORS, ISOLATORS ANDSWITCHES Donald L. White, Mendham, N..l., assignor to Bell TelephoneLaboratories, Incorporated, New York, N.Y., a corporation of New YorkOriginal No. 3,173,100, dated Mar. 9, 1905, Ser. No. 105,700, Apr. 26,1961. Application for relssue Oct. 23, 1965, Ser. No. 505,145

13 Claims. (Cl. 331-96) Matter enclosed in heavy brackets II] appears inthe original patent but forms no part of this reissue specification;matter printed in italics indicates the additions made by reissue.

This invention relates to procedures and devices for modifying acousticwave signals. It proposes a new electromechanical mechanism foramplifying or attenuating acoustic waves in solid elastic media. Thismechanism depends on the interaction of piezoelectric fields generatedin the solid by acoustic waves with electrostatic fields simultaneouslyproduced in the body by an external bias source.

Ordinarily, semiconductors are too highly conductive to support anobservable piezoelectric field. However, significant piezoelectriceffects have recently been observed in certain specially prepared, highresistance semiconductors. Certain of these, specifically ZnO, CdS andMN, are disclosed in United States Patents Nos. 3,09l,- 707, 3,093,758and 3,090,876 issued May 28, 1963, June 11, 1963 and May 21, 1963,respectively. Other semiconductors exhibiting significant piezoelectriceffects under appropriate conditions are InAs, CdSe, CdTe, GaAs, Gal andZnS.

It has now been found that an acoustic wave propagating through such apiezoelectric semiconductor medium can be effectively influenced throughinteraction of the piezoelectric field generated by the acoustic wavewith mobile carriers under the influence of an electrostatic fieldproduced in the medium by an external DC. bias source. This interactionallows for significant attenuation or amplification of the acousticsignal in response to the magnitude and direction of the electrostaticfield. Within the meaning of this invention the Word acoustic" is usedto refer to any coherent elastic wave or mechanical wave vibration ofany frequency and is intended to include those frequency rangessometimes called ultrasonic and hypersonic.

An acoustic wave travelling through a piezoelectric medium generates analternating electric field which travels at the same velocity as theacoustic wave. Since this field is non-uniform, electric currents aregenerated which tend to accumulate or bunch electrical chargesperiodically throughout the medium. The bunched charges tend toneutralize the piezoelectric field. When a DC. voltage is applied to themedium, a periodic electric field is produced by the direct currentflowing through regions in which the charge carriers have been bunched.The alternating field produced by the direct current reacts upon thepiezoelectric medium causing additional acoustic wave components. Thesemay either enhance (amplify) or diminish (attenuate) the original waveaccording to certain prescribed variables.

Forany given piezoelectric semiconductor material under the influence ofa given fixed D.C. field there is a corresponding optimum frequencygiving maximum gain (or loss). This frequency of maximum gain may berelated to the variables of the system by the formula:

fi wy (1) Re. 20,091 Reisaued Sept. 20, 1966 where w is the angularfrequency of maximum gain, p is the resistivity of the piezoelectricmaterial, a is the dielectric constant multiplied by 8.85-10 farads percm. (the permittivity of a vacuum), V is the average drift velocity ofthe carriers in the semiconductor responsive to the fixed D.C. field,and v is the velocity of sound in the medium. It is readily apparentfrom Equation 1 that the required condition to achieve amplication isthat v exceeds v Where the drift velocity is less than the velocity ofsound in the medium, attenuation occurs.

The drift velocity is dependent upon the material and the magnitude ofthe DC. field as follows:

where n is the mobility of majority carriers in the semiconductor in inthe direction of propagation of the acoustic wave.

If the drift velocity is less than v the term in the above formulae isreplaced by If the direction of the carriers drift velocity is oppositeto the direction of acoustic propagation, this term is replaced by VD 1+s Only when the components of VD is in the same direction as theacoustic propagation and is greater than v can amplification occur.Otherwise, the electric field will modify the attenuation.

Vector analysis shows that the drift velocity, v,;, in Equation 1 isactually the component of the drift velocity along the direction ofpropagation of the acoustic wave signal. Thus, it is not essential thatthe field direction coincide with the direction of the acoustic wave.

It is obvious that since the phenomenon of this invention requires theinteraction of a piezoelectric field with a DC. field, the direction ofacoustic wave propagation must be related to a piezoelectric axis of thematerial so as to generate a piezoelectric field. it is not alwaysaccurate to state that as long as the direction of propagation of theacoustic wave signal has a vector component along a piezoelectric axisof the material, a field will be generated. In certain crystalstructures opposing fields are generated which cancel each other. Thus,the direction of propagation of the acoustic wave is properly defined asany crystallographic direction which creates or produces a substantialpiezoelectric field. The direction of this field necessarily lies alongthe direction of wave propagation.

It has been found that alteration of the amplitude of acoustic waves canbe achieved whenever a drift velocity component, v is generated by theinfluence of the DC. field. However, for the purposes of non-reciprocaloperation according to the principles of this invention, the driftvelocity, V is preferably at least 5% of the acoustic velocity toprovide a preferred magnitude of non-reciprocal effect. As previouslypointed out, at the point where v v amplification occurs.

These relationships, as well as the characteristics of devices operatingaccording to the principles above set forth, will perhaps be betterunderstood when considered in conjunction with the drawings in which:

FIG. 1 is a plot of gain versus the ratio v /v for a given acousticfrequency in a given material;

FIG. 2 is .1 plot similar to FIG. 1 with a different ratio of acousticfrequency to the acoustic constants of the material;

FIG. 3 is a schematic view of an acoustic wave amplifier constructedaccording to the teachings of this inventron;

FIG. 4A is a schematic view of an oscillator utilizing the principles ofthis invention;

FIG. 4B is a schematic view of another embodiment of an oscillatorutilizing a resonant cavity;

FIG. 5 is a schematic view of an ultrasonic delay line which maysimultaneously exhibit gain;

FIG. 6 is a schematic view of an acoustic wave circulator operatingaccording to the teachings of this invention; and

FIG. 7 is a schematic view of an acoustic wave switch similar inconstruction and operation to the device of FIG. 6.

In Equation 1 it is seen that the numerator, l/pe, is generally a fixedcharacteristic of the material. As will hereinafter be more fullytreated, the resistivity, p, of the material provides a convenientmodulating mecha nism for certain semiconductor materials which arephotosensitive. Otherwise, both the resistivity and dielectric constantare invariable and the acoustic velocity, V is generally fixed. Thus,the two remaining variables are the acoustic wave frequency and thedraft velocity. Since in Equation 2 the mobility is a fixedcharacteristic of the semiconductor, the true variable is E, thestrength of the field component in the direction of acoustic wavetravel.

For general purposes the frequency level of the acoustic wave is givenby the signal desired to be modified. Therefore, in ordinaryapplications, the ratio of in Equation 1 is predetermined andinvariable.

Referring to FIG. 1 the gain (ordinate) versus v /v (absissa) is plottedfor a given ratio to to l or 2 e It is also seen that gain occurs forratios of It is also seen from the figure and from Equation 1 that themaximum gain obtainable with this material at this operating frequencyoccurs at a ratio The maximum gain achievable in complying with therelation of Equation 1 is given by:

attenuation effects in many acoustic devices are generally utilized fornon-reciprocal operation as will hereinafter be pointed out. The portionof the curve in the third quadrant of FIG. I represents the lossincurred by the acoustic signal for negative ratios of v /v is i.e.,where a drift velocity component opposes the direction of acousticpropagation. It is seen that a significant non-reciprocal effect cannotbe obtained between the points a and b corresponding to -v =5% v Hence,for nonreciprocal devices, the limit previously suggested for theminimum effective velocity ratio restricts the operation to the moreacceptable portions of the curve. For instance, a non-reciprocal deviceoperating in the material and at the frequency represented by the curveof FIG. 1 with a velocity ratio would provide a maximum attenuation inthe forward direction (point c) with substantially less loss (point d)in the reverse direction. It should be understood that operation withdrift velocity values below the limit prescribed for non-reciprocaloperation has significant devize application and is considered withinthe scope'of this invention.

The curve of FIG. 1 serves well to illustrate operating points for anyacoustic delay line exhibiting gain. A delay line operating with theratio of towof .5 pc

as characterized by the curve of FIG. I and at a V /V ratio of 1.5 wouldprovide a forward direction operating point at f showing maximum gainand a reverse direction operating point at g showing only a slight loss.Since the forward gain far exceeds the reverse loss, a signal followingmany forward and reverse reflections, as in a shear mode ultrasonicdelay line of the type well known in the art, ultimately showssignificant per transit gain by the amount point f exceeds point g. Thisgain is achieved simultaneously with the desired delay. Furthermore, thedelay time of the semiconductor medium may also be conveniently adjustedby varying the strength of the DC. field.

FIG. 2 shows an operating curve particularly adapted for non-reciprocaldevices. The coordinates are identical to those of FIG. 1. This curverepresents a ratio of towof i p6 obtained with a lower resistivitymaterial and/or using a lower operating frequency. Now if the ratio, V/V is chosen at 3, the forward operating point is m providingsignificant gain, while the reverse operating point, ,n provides maximumloss. This operating curve is well suited to non-reciprocal devices suchas isolators. As will be appreciated, all operating curves aresymmetrical about the point The greater divisions between maxima andminima are obtained with greater ratios of l/pe to w; i.e., withmaterials having lower resitivities and dielectric constants and withlower operating frequencies.

FIG. 3 shows a typical construction of an acoustic wave amplifierutilizing the principles of this invention. To the ends of body 10, asemiconductor piezoelectric material, are affixed ultrasonic transducersI1 and 12. The transducers are of the type generally used in the art. Apreferred form of transducer particularly well adapted for highfrequency operation is the depletion layer transducer disclosed incopending application, Serial No. 64,808, filed October 25, 1960. AnA.C. signal generated at 13 is impressed across transducer 11, thuscreating an acoustic signal which is transmitted through thepiezoelectric semiconductor medium to transducer 12. The outputelectromagnetic signal generated across transducer 12 by the acousticsignal is received by the voltmeter 14 through blocking capacitor 15.The D.C. field which couples with the piezoelectric field generated bythe acoustic signal is impressed across the medium 10, as shown, bysource 16.

It should be appreciated that while the device in FIG. 3 utilizes anelectromagnetic signal to generate the acoustic wave, an acoustic signalmay be injected directly into the amplifier, thus eliminating transducer11. Also, trans ducer 12 may be eliminated if the desired output is anacoustic signal. The device shown in FIG. 3 is eiiectively anelectromagnetic signal amplifier although the amplification mechanismutilizes an acoustic wave.

FIGS. 4A and 4B illustrate two forms of oscillators operating accordingto the principles of this invention. In the device of FIG. 4A anelectromagnetic signal is amplified in amplifier 20 which is essentiallyidentical to the amplifier of FIG. 3. The output is fed back to theinput by the feedback circuit shown which includes reactance 21. Theoscillator is tuned with reactance 21 and the D.C. source 22.

FIG. 4B shows an oscillator composed of an amplifier mounted in aresonant cavity 30. The amplifier consists of a piezoelectricsemiconductor body 31 with a D.C. field impressed across it by D.C.source 32 in the direction of propagation of the resonant waves throughthe body. It is seen that this oscillator includes a true acousticamplifier and no electromechanical transducers are required. At theproper frequency an electrical coupling exists between the acousticmedium and the cavity, thereby producing resonance in the cavity. Thecavity oscillations are enhanced by the amplification of the resonantfrequency through interaction with the D.C. field in the acousticmedium. The resonant wave appears at output 33.

FIG. 5 illustrates a typical ultrasonic delay line utilizing theprinciples of this invention. The delay medium 50, composed of apiezoelectric semiconductor, is similar in construction to thoseconventionally employed in the art. A detailed description of theconstruction of such a delay line and its operation appears in US.Patent No. 2,839,731. An electromagnetic signal generated at 51 is fedacross piezoelectric transducer 52. The resulting acoustic signal entersthe delay medium 50 and traverses a delay path essentially as shown, andemerges through piezoelectric transducer 53. The transducer 53 convertsthe acoustic signal back to electromagnetic energy which is indicated byvoltmeter 54. The capacitors 55 are included to block the D.C. current.Electrodes 56 and 57 bound each reflecting surface of the delay medium,and bias source 58 impresses a D.C. field between these electrodes whichhas a component lying along the path of the acoustic wave. Referringback to FIG. 1, if the drift velocity component of the carriers alongthe direction of acoustic wave propagation is chosen such that theoperating points on the curve of FIG. 1 are f for the forward directionand g for the reverse direction, the acoustic signal will experiencemaximum amplification in the forward direction with a minimumattenuation in the reverse direction as previously discussed. Thus, thisultrasonic delay line shows a significant amount of gain.

FIG. 6 shows a circulator utilizing the principles of this invention.This device has well established uses which generally involve theseparation of transmitted signals from signals being received, whereboth share the same transmission medium. The isolator describedpreviously achieves this to a limited extent but only at the expense oreven loss of one of the signals. In the device of FIG. 6 thepiezoelectric semiconductor medium 60 includes three ultrasonictransducers 61, 62 and 63, disposed as shown. It functions to maintainthe separation between a signal injected at transwuoer 61, to betransmitted along the conducting line attached to 62, from a signalreceived at transducer 63 through the common transmission line attachedat transducer 62. This effect is achieved due to the field establishedin the medium 60, by D.C. source 64 and electrodes 65 and 66. This fieldprovides a diminishing intensity from one side of the medium 60 to theother as shown. Thus, since the velocity of sound in medium 60 dependsupon the electric field intensity, the nonuniform field causes the wavesto refract. Accordingly, a wave indicated by rays la and 1b injected attransducer 61 is bent toward transducer 62. However, a wave indicated by2a and 2b, injected at 62 is infiuenced by a field of opposite directionand is bent toward transducer 63. As is seen, this device isnon-reciprocal in that no acoustic wave can retrace its prior path.Appropriate operating points for operation of this device would be pointI (FIG. 1) for the direction between 61 and 62, and corresponding pointg (FIG. 1) for the reverse direction. Using these points the signalbeing transmitted from transducer 61 to transducer 62 would besignificantly amplified while the signal received from the transducer 62to the receiver attached at transducer 63 would be only slightlyattenuated. This circulator, accordingly, functions additionally as anamplifier of the signal to be transmitted.

FIG. 7 shows a switching device operating according to the principles ofthe circulator of FIG. 6. The device construction is similar to that ofFIG. 6. Semiconductor body 70 carries three piezoelectric transducers71, 72 and 73 disposed as shown. The transducer 73 is essentiallyopposite to the transducer 72. D.C. source 74 and'electrodes 75 and 76establish the desired field. In operation an acoustic signal generatedat transducer 72 normally traverses the path indicated by rays 3a and 3band is received at transducer 73. However, upon application of the D.C.field at source 74, the wave is refracted and assumes a directioncorresponding to rays 4a and 4b and is received at transducer 71.

The following examples are illustrative embodiments of particularmaterials and procedures for obtaining devices of this invention. Eachexample employs the operating curve of FIG. 1. Using these examples,devices may be constructed to achieve any of the operating points on thecurve of FIG. 1. These operating points, when properly chosen aspreviously discussed, may be used for any of the devices describedherein.

EXAMPLE I A single crystal of GaAs, having a resistivity of 1000ohm-cm., is cut with a 3 mm. cross section and 2 cm. length with thelength extending in the (111) crystallographic direction. Piezoelectricdepletion layer transducers are then formed in each end in the mannerfully described in application Serial No. 64,808, filed October 25,1960. The device construction is identical to that of FIG. 3. To obtainthe ratio of to w of 0.5 p

as the curve of FIG. 1 represents, the operating voltage is calculatedfrom Equation 1. In this example the GaAs has a resistivity of 1000ohm-cm. and a dielectric constant of l 1. Thus, the value l/pe inEquation 1 is set at 10lsec. Therefore, to obtain a ratio to m of 0.5 e

the operating frequency (angular) w=2 l0 rad/sec. or 320 megacycles persecond. From Equation 1 the ratio of drift velocity to acoustic velocityis calculated as:

From the velocity of sound in this material, 5.6)(10 cm./sec., the driftvelocity is calculated as 8.4)(10 cum/sec. Equation 2 relates the driftvelocity to the rc quircd electric field as:

D F Here the entire velocity componcnt lies in the direction of theacoustic wave propagation (P10. 3). Thus, Equation 2 becomes:

VD=ME where E is the field, and u, the mobility of carriers in thismaterial, is

cm. 4,000 Van a. The field E is then:

mant as 4,000

= 210 volts/cm.

Thus, the transducers at each end of the crystal are adjusted for anoperating frequency of 320 me. The DC. source, supplying the 420 voltpotentials required for the 2 cm. sample, is connected as indicated inFIG. 3. The operating points on the curve of FIG. are f and g. The highfrequency signal is impressed across the input transducer II, in FIG. 3,and detected across output transducer 12. The output signal is amplifiedapproximately 20 db. This device, therefore, operating at thisdesignated frequency and DC. potential, provides a gain of approximatelylOdb/cm.

EXAMPLE II A single crystal of CdS is cut with a cross-section of 1 mm.x 1 mm. and a length of 2 mm. with its length extending in thec-direction. The device structure is the same as that in Example I. Thismaterial, with a resistivity of 300 ohm-cm. has a l/ps value of 3.9-lO/sec. Hence, to achieve a ratio of into f 2 pi which is the basis forthe curve of FIG. 1, the operating frequency, w, is 7.8- 10 radians/sec.or f=1,250 me. In this material and crystallographic direction v =4.5-l0cm./see. For a ratio of drift velocity to the sound velocity of 1.5 (ascalculated in the previous example) v tnust equal 6.7- cm./sec. For CdS,=300 cm./ volt sec., the field required to give this drift velocity(calculated from Equation 2) is 440 volts.

The square of the coupling coefficient :2 (a) for this CdS is .07, l/Ain this example is 3.6-l0 The gain for this device is 65 db or 330db/cm.

A further control parameter, as previously pointed out, and which isapparent from Equation 1, is the resistivity of the material. Since somesemiconductors, for instance GaAs and CdS, are photosensitive, i.e.,their resistivities vary with the intensity of incident illumination,Thus, an appropriate variable light source, with which the art is wellacquainted, may be employed to vary the resistivity and attendant gain.A more thorough treatment of the variation of resistivity withillumination in piezoelectric semiconductors will be found inapplication Serial No. 23,441, filed April 20, 1960. Usefulresistivities for any of the semiconductor materials previouslyindicatedas appropriate for this invention are in the range of l ohm-cm. to I0ohm-cm. As seen from Equation 1, the lower resistivity materials providehigher frequency devices.

Whereas single crystal mediums are preferred, polycrystallinepiezoelectric semiconductors are acceptable.

The frequency range in which the devices of this invention are adaptedto operate is from 200 mc. to over ltlti kmc. However, at highfrequencies, since the carrier bunches corresponding to compressions anddilations L E l'u WI 1 v8 where i is the frequency threshold at whichcarrier diffusion reduces the gain by 50%, V is the velocity of sound,and D is the diliusion cociiicient given by:

where is the carrier mobility in volt-sec.

T is the absolute temperature in K., k is Boltzmazts constant=1.38-10ergs/ K., and q is the electron charge=l.6-1O- coulombs.

For zinc oxide and cadmium sulfide, at v =:2v i is approximately 10,000mc./sec. at room temperature; since these materials are so stronglypiezoelectric, significant amplification may be obtained at frequencieswell in excess of this value.

It will be obvious to those skilled in the art that in transmitting veryhigh frequency electromagnetic signals, wave guides or similar handlingof the signal must be employed. The figures are to be consideredschematic in this regard, and when V.H.F. and microwave frequencies arecontemplated, the wires appearing in the figures are to be understood asindicating the necessary transmission structures as are well known inthe art.

The foregoing examples are offered as exemplary of the multitude ofpossible device designs which depend upon the basic teachings of thisinvention and are not to be construed as limiting the invention. Variousother modifications and embodiments will become apparent to thoseskilled in the art. However, all such devices, which are characterizedin whole or part by the basic phenomenon through which this inventionhas advanced the art, are properly considered within the spirit andscope of this invention.

What is claimed is:

1. An electromechanical device comprising a unitary body having bothpiezoelectric and semiconductor properties, means for propagating anacoustic wave signal through said body thereby generating a significantpiezoelectric field, and means including a DC bias source forsimultaneously establishing a DC. field in said body, said field havinga magnitude and direction such that the drift velocity of the carriersresponsive to said field has a velocity component along the axis definedby the direction of acoustic wave propagation whereby the carriers ofsaid D.C. field interact with the said acoustic wave in a manner so asto modify the amplitude of the acoustic wave.

2. The device of claim 1 wherein said drift velocity component is atleast 5% of the velocity of the acoustic signal.

3. The device of claim 1 wherein the piezoelectric semiconductor isselected from the group consisting of GaAs, GaP, ZnO, CdS, InAs, ZnS,CdTe, AlN and CdSe.

4. The device of claim 1 wherein the component of the said D.C. fieldalong the said axis opposes the said acoustic wave signal.

[5. The device of claim 1 wherein the means for propagating an acousticwave signal through said body is a resonant cavity] 6. An acoustic waveamplifier comprising a unitary body having both piezoelectric andsemiconductor properties, means for propagating an acoustic wave signalthrough said body thereby generating a significant piezoelectric fieldand means including a DC. bias source for simultaneously establishing aDC. field in said body, said field having a magnitude and direction suchthat the drift velocity of the carriers responsive to said field has avelocity component in the direction of acoustic wave propagation whichis greater than the velocity of the acousttc signal whereby the saidcarriers of the DC. field interact with the acoustic wave to increasethe amplitude of the acoustic wave.

7. The amplifier of claim 6 wherein the means for propagating theacoustic wave signal through said body includes an ultrasonicpiezoelectric transducer attached to said piezoelectric semiconductor atthe point of injection of the acoustic wave.

8. The amplifier of claim 7 additionally including an ultrasonicpiezoelectric transducer in combination with said piezoelectricsemiconductor for reconverting said acoustic signal after amplificationback to an electrical signal.

9. The amplifier of claim 6 wherein the means for propagating theacoustic wave signal produces a signal frequency above 200 me.

10. The device of claim 6 wherein the piezoelectric semiconductor bodyadditionally constitutes an ultrasonic delay medium whereby the devicesimultaneously functions as an ultrasonic delay medium and an acousticwave amplifier.

11. An isolator comprising a unitary body having both piezoelectric andsemiconductor properties, means for propagating an acoustic wave signalthrough said body thereby generating a significant piezoelectric fieldand means including a DC. bias source for simultaneously establishing aDC. field in said body whereby the DC. field interacts with the acousticwave to modify the amplitude of the acoustic wave in a nonreciprocalmanner.

12. An ultransonic circulator comprising a unitary body having bothpiezoelectric and semiconductor properties, transmitting and receivingmeans afiixed to a first surface of said body for transmitting a firstacoustic signal through said body and receiving a second acousticsignal, receiver means afiixed to a surface opposite to the said firstsurface of said body for receiving said first acoustic signal andtransmitting means affixed to said opposite surface of said body andspaced from said receiving means for transmitting a second acousticsignal through said body in a direction substantially opposite to thedirection of said first acoustic signal such that said second signal isreceived by said receiving and transmitting means on said first surface,and means associated with said body for establishing a DC. field havinga diminishing intensity across said body in a direction approximatelynormal to the direction of propagation of the said acoustic signals.

13. An ultrasonic device comprising a unitary body having bothpiezoelectric and semiconductor properties, a first piezoelectrictransducer attached to a first surface of said body, an additional pairof piezoelectric transducers spaced from each other, each attached tosurfaces of said body essentially opposite to said first surface anddefining two discrete paths between each of said pair of transducers andsaid first transducer, and means associated with said body forestablishing a DC. field along one of said discrete paths which has afield value different from the other of said discrete paths.

14. An oscillator comprising a unitary body having both piezoelectricand semiconductor properties, said body enclosed within a cavity adaptedto support high frequency electromagnetic waves, means including a DC.bias source for establishing a D.C. field in said body and forsimultaneously generating an acoustic wave signal in said body with anaccompanying significant piezoelectric field, said D.C. field having amagnitude and direction such that the drift velocity 0 the carriersresponsive to said field has a velocity component along the axis definedby the direction of acoustic wave propagation whereby the carriers ofsaid D.C. field interact with the said ac0ustic wave in a manner so asto modify the amplitude of the acoustic wave, and electrical outputmeans for coupling electromagnetic energy from the said cavity.

References Cited by the Examiner The following references, cited by theExaminer, are of record in the patented file of this patent or theoriginal patent.

UNITED STATES PATENTS 2,794,863 6/ 1957 Roosbroeck. 2,839,731 6/1958McSkimin. 2,898,477 8/1959 Hoesterey. 3,012,211 12/1961 Mason. 3,090,8765/1963 Hutson.

OTHER REFERENCES Hutson: Piezoelectricity and Conductivity," PhysicalReview Letters, vol. 4, No. 10, May 1960, pp. 505-507.

ROY LAKE, Primary Examiner. F. D. PARIS, Assistant Examiner.

